1. Field of Invention
The present disclosure relates generally to metal deposition systems and, more specifically, to systems and methods for the detection and/or rectification of conditions caused by impurities in metal evaporation sources used in electron-beam metal evaporation/deposition.
2. Discussion of Related Art
Various steps in the processing of semiconductor wafers to form microchips for use in electronic devices involve the deposition of one or more layers of metal on the semiconductor wafers. These metal films are used to form, for example, metal contacts or conductive pathways. Metal films are deposited on semiconductor wafers generally through the use of either chemical vapor deposition (CVD) systems or physical vapor deposition (PVD) systems. PVD systems are generally divided into sputtering systems and evaporation systems.
In sputtering systems, an energetic beam of ions, for example, argon ions, is directed at a metal target in a vacuum chamber. The energetic ions knock metal atoms free from the target. The freed metal atoms travel through the vacuum chamber and deposit on one or more wafers also present in the vacuum chamber.
In evaporation systems (also referred to herein as evaporation/deposition systems), a metal source (also referred to herein as a metal slug) is heated in a vacuum chamber, maintained at about 10−7 Torr in some systems, until the metal melts and atoms evaporate from the metal source. The metal source may be heated by any of a number of methods, including, for example, resistive heating or by directing an electron-beam into the metal source. The metal atoms evaporated from the metal source travel through the vacuum chamber and deposit on one or more semiconductor wafers also present in the vacuum chamber.
During the deposition of metal onto a semiconductor wafer, in accordance with some semiconductor manufacturing processes, the semiconductor wafer may be covered by a blocking material, conventionally referred to as a “mask,” on areas of the wafer in which it is desired that a metal film not be formed. The mask may be formed from, for example, a patterned layer of photoresist (also referred to herein as “resist”). Open areas in the mask are formed where it is desired that the metal film be deposited onto the wafer. These open areas may be formed by, for example, applying a layer of photoresist to a wafer and exposing the photoresist to light which has passed through a lithography mask including a pattern desired to be formed in the photoresist. The photoresist exposed to the light becomes polymerized. A subsequent development step chemically removes non-polymerized photoresist. The remaining photoresist is baked to remove volatile chemicals. Desirably, the remaining photoresist is polymerized, but not cross-linked, i.e., hardened. Aspects and embodiments of the methods and apparatus disclosed herein are not limited to semiconductor manufacturing processes using any particular mask formation process.
After deposition of the metal film, the mask is removed, taking with it any metal that was deposited on the mask, a process known as metal lift-off. What is left behind is a metal film formed in the areas on the semiconductor wafer that were not blocked by the mask.
In some semiconductor manufacturing processes, metallized wafers are put through a wet strip process in a solvent such as N-Methyl Pyrrolidone (NMP) or ethylene glycol to dissolve photoresist that was used as a mask to define the desired metallization pattern, liftoff the unwanted metal(s), and to form a desired portion of an electrical circuit.
Most available photoresists can be cross-linked if exposed to excessive heat or light. Cross-linked or hardened photoresist will not dissolve completely in the normal wet strip chemicals used in some manufacturing processes. A photoresist residue will thus remain on a wafer after the stripping process if the photoresist on the wafer became cross-linked prior to the stripping process. Although the photoresist residue can usually be removed by reworking using more aggressive wet and/or dry strip processes, the additional rework steps negatively impact the production flow and manufacturing schedule.
Further, if contamination present on a semiconductor wafer, such as photoresist residue or nodules from metal “spitting,” discussed below, are not detected on the wafer, this contamination may lead to further problems with downstream processing steps. Such problems may include, for example, poor adhesion or planarity of subsequently deposited layers. These problems may result in a reduction in line yield (the amount of wafers that are not scrapped during manufacturing) and/or die yield (the amount of functional devices per wafer formed in the manufacturing process). Undetected contamination may also lead to reliability problems including failure of a device in the field.